1. Field of the Invention
The present invention relates to a resin composition, and more particularly to a polymer composition which is excellent in softness, transparency and moldability or shapability.
2. Description of the Prior Art
Molding resin materials having excellent softness and transparency recently used include block copolymers which comprise an intermediate block (about 5,000 to about 125,000 in number average molecular weight) of a hydrogenated conjugated diene polymer, such as ethylene-butylene copolymer or ethylene-propylene copolymer, and terminal blocks (about 10,000 and about 300,000 in number average molecular weight) of an aromatic vinyl compound copolymer, such as polystyrene (U.S. Pat. Nos. 4,140,162 and 4,588,777).
Such block copolymers are obtained usually by the hydrogenation of block polymers having the structure of (vinyl aromatic polymer block)-(conjugated diene polymer block)-(vinyl aromatic polymer block), such as polystyrene-polybutadiene-polystyrene structure or polystyrene-polyisoprene-polystyrene structure. The degree of hydrogenation of the conjugated diene polymer block is at least 90%. Although it appears likely that the aryl of the vinyl aromatic polymer blocks will also be hydrogenated during the hydrogenation, the degree of hydrogenation thereof is less than 10% at its highest. Thus, it is thought that only the conjugated diene polymer block is substantially subjected to hydrogenation.
However, these block copolymers are not satifactorily moldable by extrusion or injection when singly used. For example, the block copolymer, when in the form of particles, powders or flakes, is difficult to make into molding pellets or is in no way extrudable into sheets or tubes. Actually, therefore, such block copolymers are used as spread in the form of a flowable organic solvent solution, or as blended with some other resin such as polyolefin, or as mixed with plasticizers, oils, etc. into a moldable composition.
Nevertheless, the spreading of the copolymer in the form of a solution needs a cumbersome procedure and further requires much labor and cost for completely removing the residual solvent. Blending with other resin or addition of plasticizer, oil and the like alters the properties such as transparency, softness and strength, not infrequently giving a composition which is unsatisfactory for the contemplated use.
Accordingly, it has been desired to provide a resin material which retains the excellent properties of the block copolymer in respect of transparency, softness, strength, etc. and which nevertheless has high moldability or shapability.
The present invention has been accomplished in view of these problems.
When a block copolymer (a) having terminal blocks in an amount of about 10 to about 20% by weight based on the copolymer was used alone, it was actually impossible to prepare pellets from this copolymer by a continuous operation because when pellets were to be prepared by extrusion, the copolymer exhibited high tackiness and low viscosity, rendering the resulting strands difficult to take up. Further even if the copolymer was extruded through a T-die or tubular die, it was impossible to continuously wind up the extrudate. Thus, it was impossible to extrude the copolymer directly into sheets or tubes. While a block copolymer (b) which comprises terminal blocks in an amount of about 25 to about 35% by weight based on the copolymer (b) was used alone, problems were also encountered with this copolymer. When the copolymer was extruded for the preparation of pellets, the resulting strands were markedly rough-surfaced, developing numerous large crevices in an extreme case, so that it was actually difficult to prepare pellets. Accordingly, it was totally impossible to extrude the copolymer into tubes or sheets.
We have conducted intensive research and various experiments in order to improve the moldability or extrudability of the foregoing block copolymers and consequently discovered the surprising fact that whereas the block copolymer (a) and the block copolymer (b) fall into the category of the foregoing block copolymers and were not actually extrudable when singly used, these copolymers (a) and (b) exhibit excellent moldability or extrudability when used in combination. The present invention has been accomplished based on this finding.